Measuring devices which are implemented for progressive tracking of a target point and coordinate position determination of this point can be generally summarized, in particular in conjunction with industrial surveying, under the term laser trackers. A target point can be represented in this case by a retroreflective unit (for example, cube prism), which is targeted using an optical measuring beam of the measuring device, in particular a laser beam. The laser beam is reflected in parallel back to the measuring device, wherein the reflected beam is acquired using an acquisition unit of the device. An emitting or receiving direction of the beam is ascertained in this case, for example, by means of sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. In addition, a distance from the measuring device to the target point is ascertained with the acquisition of the beam, for example, by means of runtime or phase difference measurement.
Laser trackers according to the prior art can additionally be embodied having an optical image acquisition unit having a two-dimensional, light-sensitive array, for example, a CCD or CID camera or a camera based on a CMOS array, or having a pixel array sensor and having an image processing unit. The laser tracker and the camera can be installed one on top of another in this case, in particular in such a manner that the positions thereof relative to one another are not variable. For example, the camera is rotatable together with the laser tracker about its substantially vertical axis, but is pivotable up-and-down independently of the laser tracker and is therefore arranged separately from the optic of the laser beam in particular. Furthermore, the camera—for example, as a function of the respective application—can be embodied so it is only pivotable about one axis. In alternative embodiments, the camera can be installed in an integrated construction together with the laser optic in a shared housing.
With the acquisition and analysis of an image—by means of image acquisition unit and image processing unit—of a so-called measuring aid instrument having markings, the relative location of which to one another is known, an orientation in space of an object (for example, a probe) arranged on the measuring aid instrument can be concluded. Together with the determined spatial position of the target point, furthermore the position and orientation of the object in space can be precisely determined absolutely and/or in relation to the laser tracker.
Such measuring aid instruments can be embodied by so-called scanning tools, which are positioned with their contact point on a point of the target object. The scanning tool has markings, for example, light spots, and a reflector, which represents a target point on the scanning tool and can be targeted using the laser beam of the tracker, wherein the positions of the markings and the reflector in relation to the contact point of the scanning tool are precisely known. The measuring aid instrument can also be, in the way known to a person skilled in the art, a handheld scanner, which is equipped for distance measurement, for contactless surface surveying, for example, wherein the direction and position of the scanner measuring beam used for the distance measurement in relation to the light spots and reflectors, which are arranged on the scanner, are precisely known. Such a scanner is described, for example, in EP 0 553 266.
For distance measurement, laser trackers of the prior art have at least one distance meter, wherein it can be implemented as an interferometer, for example. Since such distance measuring units can only measure relative distance changes, in addition to interferometers, so-called absolute distance meters are installed in current laser trackers. For example, such a combination of measuring means for distance determination is known by way of the product AT901 from Leica Geosystems AG. The interferometers used in this context for distance measurement primarily use HeNe gas lasers—as a result of the long coherence length and the measurement range thus made possible—as light sources. The coherence length of the HeNe laser can be several hundred meters in this case, so that the ranges required in industrial metrology can be achieved using relatively simple interferometer constructions. A combination of an absolute distance meter and an interferometer for distance determination using an HeNe laser is known, for example, from WO 2007/079600 A1.
In addition, a divergence of the received measurement beam from a zero position is ascertained on a sensor in modern tracker systems—increasingly as a standard feature. By means of this measurable divergence, a position difference between the center of a retroreflector and the point of incidence of the laser beam on the reflector can be determined and the alignment of the laser beam can be corrected or tracked as a function of this deviation in such a manner that the divergence on the sensor is decreased, in particular is “zero”, and therefore the beam is aligned in the direction of the reflector center. By way of the tracking of the laser beam alignment, progressive target tracking of the target point can be performed and the distance and position of the target point can be determined progressively in relation to the measuring device. The tracking can be implemented in this case by means of an alignment change of the deflection mirror, which is movable by a motor and is provided for deflecting the laser beam, and/or by pivoting the targeting unit, which has the beam-guiding laser optic.
The described target tracking must be preceded by coupling of the laser beam to the reflector. For this purpose, in addition an acquisition unit having a position-sensitive sensor and having a comparatively large field of vision can be arranged on the tracker. Moreover, additional illumination means are integrated in devices of this type, using which the target or the reflector is illuminated, in particular using a defined wavelength, which differs from the wavelength of the distance measuring means. In this context, the sensor can be implemented as sensitive to a range around this specific wavelength, for example, to reduce or completely prevent external light influences. By means of the illumination means, the target can be illuminated and an image of the target having illuminated reflector can be acquired using the camera. By way of the imaging of the specific (wavelength-specific) reflection on the sensor, the reflection position in the image can be resolved and therefore an angle in relation to the acquisition direction of the camera and a direction to the target or reflector can be determined. An embodiment of a laser tracker having such a target search unit is known, for example, from WO 2010/148525 A1.
As a function of the direction information which is thus derivable, the alignment of the measuring laser beam can be changed such that a distance between the laser beam and the reflector, to which the laser beam is to be coupled, is reduced. As a result of the offset of the optical sensor axis defined by the sensor and the axis along which the measuring laser beam propagates, the alignment of the beam on the target by means of the sensor-based direction determination to the target and therefore the coupling cannot be performed in a direct step. For a stationary target, multiple iteration steps are required for this purpose, each having a measuring operation (renewed determination of a direction to the target using the sensor) for the approach of the laser beam. Such an approach method therefore has the disadvantage that searching for and targeting the target is a time-consuming operation (since it is iterative) and the searching is not robust and unambiguous, in particular in the event of a relative movement of the target to the sensor. Furthermore, in the event of a movement of the target relative to the laser tracker, an approach of the laser beam to the target can no longer succeed, since a deviation between the target detected by means of the sensor and the laser beam changes progressively in this case. An iterative approach of the beam to the target thus cannot be performed as a result of this sustained deviation change during the movement of the target. Every iteration step having a renewed acquisition of a reflection corresponds in this case to a first such measurement on a (new) target. In general, a large disadvantage of such targeting systems therefore additionally results, that stationary targets can only be targeted using a relatively very large time expenditure and the direct targeting of moving targets is impossible.